1. Field of the Invention
This invention relates generally to an electrode structure for a glow discharge system and more specifically to an electrode structure in a cathode glow discharge system used for nitride hardening of metals where an insulating shroud is placed between the two electrode plates.
2. Discussion of Background
The hardening of metal is an important technology in many large industries including the manufacture of communications mechanisms, petroleum production and defense systems. One type of hardening which has had great success for many years is the nitride hardening heat treatment process of metals. In this process, the electrode structure of the glow discharge system must be carefully designed in order to prevent problems from arc ignition. Traditionally, an air gap of approximately 1 mm has been used as a shield to prevent an unstable glow discharge from igniting an arc unexpectedly on the electrode structure. While the traditional shielding has been of some use, it has not been totally effective in preventing these arcs.
Some of the early work done in this field was performed by Bernhard Berghous who produced many inventions relating to this field at least as early as the 1930's. Some of these devices are seen in U.S. Pat. Nos. 2,200,909, 2,219,614, 2,219,615, 2,1231,104, 2,266,735, 2,371,278, British Pat. Nos. 520589 and 526527 and German Pat. Nos. 1,044,984, 1,058,806 and 1,156,624. In many of these devices, the traditional shielding method of providing an air gap or a vacuum gap between electrodes is seen. However, as pointed out above, this type of shielding does not always prevent arcs from occurring.
The prior art electrode structures using this form of shielding have been used for several years and it is clear that such arcing has not been completely avoided. Since a great deal of current can be produced by such an arc discharge in a very small area, the density of such current can reach to thousands of amperes. Due to this high current density, great destruction can result to the extent that the furnace used in the process may actually destroy itself. Thus, it is highly desirable to avoid such arcs.
During the nitride hardening process, the voltage drop on the surface of the cathode may be increased to avoid such an arc discharge. As a result, a large number of positive ions formed by this strong electric field bombard the molecules in the region near the cathode at high speed, which causes a primary ionization in the region near the cathode. Positive space charges which are formed in this ionization process in the region away from the cathode produce a very strong voltage gradient toward the cathode. Due to this very strong electric field, the cathode voltage drop is decreased which makes the air gap shield mentioned above to start glowing and fail in its protecting function. As a result, a strong arc discharge is inevitably induced. Thus, damage to the furnace is also inevitable.
Another disadvantage suffered by the prior art devices is overheating caused by the glow lights and the reflected light on the metal parts. As a result of all this light, the frame absorbs a great deal of heat and tries to dissipate the energy. As a result, the electrode structure tends to overheat.